This essay examines the nature of being an expert witness as a psychiatrist or a psychologist. The critiques of the psychiatrist as expert in the criminal justice systems produced by Michel Foucault, and Robert Musil provide a starting point for this study. Today's mental health experts working in the criminal justice field have an increasing burden of responsibility as a result of their wider role, and potentially greater power to harm. This requires an awareness of the dangers of misusing that power in part from misunderstanding its source. The expert's legitimacy stems from the knowledge they mediate. In psychiatry, we have an important, but limited, body of relevant quantitative scientific data coupled to a mass of qualitative observations with which we fill the gaps and construct our professional narratives. Confusing the science with the poetry makes us foolish and even more open to manipulation by authorities pursuing legal, governmental and populist agenda. The choices that face us lie between being a 'forensicist', tied to the legal discourse, or a being a physician, committed to mediating medical science and clinical experience. The middle ground is a slippery slope in the direction of the power of the criminal justice system.